WASHINGTON, June 26 -- Eighteen Americans spent weeks in a specialized treatment unit at Nebraska Medicine in Omaha, completing a 42-day medical monitoring protocol, the full incubation window for Andes virus, the only form of hantavirus known to pass from person to person. They had no symptoms. None developed the disease. On June 21, the Centers for Disease Control and Prevention confirmed that every one of the 18 had cleared the monitoring window without incident.

The announcement marked the end of the American chapter of an outbreak that, for nearly three months, had placed health authorities across 32 countries on alert and forced contact tracers to track more than 600 people on five continents. On June 24, the CDC formally wound dow...